The present invention relates to a method estimating the gas pressure upstream or downstream of a complex restriction, and more particularly to a method of estimating the gas pressure in an engine exhaust manifold upstream of an engine exhaust system.
An accurate indication of the gas pressure in the exhaust manifold of an internal combustion engine is required in order to accurately and reliably perform control and diagnostic functions, including fuel injection, Exhaust Gas Recirculation (EGR) valve control and Air Injection Reaction (AIR) control. Although the exhaust manifold gas pressure may be measured directly with a dedicated sensor, most automotive manufacturers have relied on an estimate of the pressure in order to save the cost of the sensor. For example, the pressure can be estimated using a variable adjustment or offset that is heuristically determined in relation to engine operating parameters, such as engine speed. However, the accuracy of the estimate tends to vary with operating conditions, particularly the variation in barometric pressure associated with altitude changes and the variation in the exhaust manifold gas temperature. Alternatively, the pressure can be estimated by iteratively solving a dynamic model of the engine combustion process. However this approach requires significant computational capability, and the accuracy of the estimated pressure tends to deteriorate when the exhaust manifold pressure is near barometric pressure. Accordingly, what is needed is an estimation method for use in production applications that is simple to implement and that provides a more accurate estimation of the exhaust manifold gas pressure.
The present invention is directed to an improved method of estimating the gas pressure in the exhaust manifold of an internal combustion engine by characterizing the engine exhaust system as a restriction, and estimating the exhaust manifold pressure as the gas pressure upstream of the restriction based on calibrated characteristics of the exhaust system and known characteristics of exhaust gas flow through the exhaust system. The estimation is based on a mathematical model that relates the mass flow of gas through the engine exhaust system to the exhaust manifold pressure (i.e., the upstream pressure), the barometric pressure (i.e., the downstream pressure) and the exhaust manifold gas temperature. An estimate of a pressure ratio across the exhaust system is calibrated based on the model parameters, and the exhaust manifold pressure is determined by applying the barometric pressure to the estimated pressure ratio. In a preferred embodiment, the mass flow of gas through the engine exhaust system is estimated using other engine gas flow estimates, including the inlet mass flow and the EGR valve mass flow. In a broader sense, the present invention provides a method of estimating the pressure upstream or downstream of a restriction passing a known mass air flow, given the mass air flow and its temperature, and one of the upstream or downstream pressures.